Nitric acid and bisulfite digestion of fibrous vegetable material



Patented May 20, 1952 rnrsm @FFICE 1 Various methods of treatment df rvegetable materials by'me'ans-of nitric racids in combination with suitable ilye's, have already een suggested. But they are very expensiveiibothzlin'wiewtof:the

high consumption of .nitric acid of athe high costs caused by the recovery :of' the sacid retained by the vegetable rmatter that leaves the bath oftreatment and which:is'icarriedwalong by the washing waters. I

The object'of the'present'inventiontis ftofprovide a method applicable to all fibrouswegetable materials and which permits' of icbtainingriazcon siderable economy over 'iknown .LmethQdS, towing in particular to the formation' of some valuable chemical products.

My method includes a successiomofioperations intended:

a. To subject the vegetable imaterial itc the action of a nitric" acid solution;

b. To neutralize lby means-of fa baseithefnitric solution retained by the vegetable matter;

c. To disincrust, by =means' of abisulfite .Iye, the non-cellulosic elements'of the 've'getable material, made soluble by -the precedingifztreatment.

a. According to my invention,'atire vegetable material, suitably cut into piecea -sis itreated fflrst by a nitric acid solution the strength .61 which varies according to the-nature :of the vegetable material, and to the pressure, ltemperaturerand duration conditions ofLthetreatment. .1:Eor:practical purposes the 'operationiis ..'carried ;out=;at atmospheric pressure and 1 in'ithi's case fthe tem perature must range from-SILBCP.Qsatimost.

In order to obtain thefhighest;yieldiasiwelleas the best quality of icellulosezit; "advisabietzto work at a temperature .as' .low possiblegzin view of the duration ofa cooking,whichashould be short. The acid solution should be i.'0f 'a strength ranging from 2 8 1%. iior Za-YVBg'GEBbIB material of low density,v very-:finely divided-rand consequently capable tof being :tqui'cklyrimpregmated with acid, to 20 :B.:..for::a vegetablamaterial of high density and Whiclrziszmorexidiflicult to impregnate. By -working iin ithis way,iit'ils' possible to obtain :ia:satisfactory:cookingximless than one hour of treatment :thefivegetable: ma terial being heated during ":this i time tirom" ordinary temperature to a maximum .temperature of70-75 C.

This treatment mayeventuallyi besprecedediby another treatment 'inten'ded' to ifacilitate" or' lt'o improve the action of-the subsequentztreatments-z Washing in hot or cold 'water, -in -st'eam,i{inan alkaline lye etc. 'It'may alsoibe icllowedzby theiorder ito 'a'eliminate ?the :maximum "amounts of treatment solution and ofsolubleproducts.

Eb; PAs-:;soon.- as :the desired ;maximumtemperature is reached, the acid retained (both by .:im- :pr'egnation: and by: superficial physical adhesion) bytheivegetablematerial freed from its acidsolu- 'fiOIl,i'iS-f1!6l1t1-311Zfid by an alkaline .or alkali earth base. ThiSiSeCOI1d i'treatment'ccan be carried out atiatmosphericrpressure either in the cold or the ihotsstate; vsits duration is determined by the necessity of fully neutralizing the acid and it depends,..'as'aatmatter:of fact, upon then'atureof thewegetalcile:material,.the temperature of treatmenttand :the concentration of the nitric .so1u tionthatiisiused. This concentration will Abe such that after-neutralization of the acid, :the bathrisztolbe:alkaline. In these conditions the duration .of i the :treatment at a temperature of abo11t50-100 C. ranges from to 60.minutes according 'ito the vegetable material that is treated.

The base that is employed is either soda or lime. I then obtainas main salts A .soluble nitrate-of sodium or calcium;

.Anoxalateproduced byzthe actionof the base on oxalic. acid resulting from reactionof nitric acid .upomthe incrustations of the vegetable ma- .teriala-and which, according to thenature of .the base, will be either a soluble sodium oxalate or an insoluble calcium oxalate. v

;.'I.his. second treatment may also befollowedby washing operations in hot or'cold .water or in steam with-a view to. eliminating the maximum amount of '-treatment solution and of :soluble products.

we. Freed from its :alkaline lye the vegetable materialstillzcontains (due to physical-adhesion) awsmall amount ofbase and of the salts'that havewbeenformed. 3

,:'It .then undergoes the .third treatment' by means of a bisulfite lye, which is preferably of .alk alieearth :nature if the neutralization base was =alkaline, :or :alkaline if "the :base ,was .,of alkalieearthznature. -'I'his. disincrustation;treatmentamay be: carriedlout either at. atmospheric pressure L01 :at 5'9, tihigher pressure either :in the hot or :thezcold Jstate; its duration rdepends uponztthe degree of cookingof the vegetable material "after .the 'J-first treatment,- and upon vthe temperature and pressure of treatment and'the concentration of theirlyethat :isutilized. iThis concentrationlwillgbexsuch that the lye: canre- 'act uponttheswhole-of the nitrates thataare carriediialong andsidissolve: .thez incrustations vof. the

same Washing ao eratigntm :watermurssteami:m:.;u weg'eta-ble material, 'TWhiGh .?have: been .anitrated The insoluble calcium oxalate produced dur-' ing the second treatment is eliminated either physically through known methods: sanding,

centrifugating etc., or chemically by washing. with a slightly acidulated solution for instance of nitric acid, which transforms calcium oxalate into oxalic acid and a soluble salt such for instance as calcium nitrate; this washing may be followed by a washing in water or with a dilute solution of a base such as soda for neutralizing the nitric acid that might remain in the paste. I may also combine the physical and chemical means.

The lyes that have been used for these three treatments may, after being restored to their strength, be again utilized. As they are being reemployed, the impurities coming from the reactions, in particular calcium oxalate; concentrate and their elimination by decantation or otherwise will be facilitated.

Industrial exploitation of the method which has just been described gives rise, as above stated, to the formation of very interesting products.

' In thev course of the second treatment, the basic reaction: soda or lime upon the nitric acidretained by the vegetable material causes the formation of sodium or calcium nitrates and oxalates. Under the action of the bisulfites (third treatment). these nitrates are transformed according to the reversible reaction:

Ca (N03) 2+2NaHSO3:2NaNOa +Ca(HSO3) z 1 For instance,'if the base that is employed is lime, thereaction yields calcium nitrate which is subsequently transformed by the action of sodium bisulfite into sodium nitrate, which can be'sold. Calcium bisulfite-remains which can be stored up to be utilized in the course of subsequent treatments.

As a matter of fact, when thereis a sufiicient stock of calcium bisulfite, it is possible to treat (second treatment) the vegetable material cooked with nitric acid,-no longer with lime, but with soda which will give sodium nitrate on which the precedingly manufactured and stored calcium bisulfite will be caused to react during the third treatment; I will thus obtain calcium nitrate, which can be sold. The sodium bisulfite manufactured at this time may be stored up for use in subsequent treatments.

Therefore it will be seen that, except for the small amounts necessary for compensating those consumed during the treatment, the first amount of bisulfite can be constantly reused, under one form or the other; it sufiices for this purposeto. make use alternately of calcium and sodium for the second treatment. On the other hand, it is known that the nitrates of these bases constitute a high value fertilizer.

Thus, the method according to my invention no longer includes costly recuperations of nitric acid carried away by the washing waters. There is but to store up the bisulfites either of sodium or of calcium and to purify them at intervals of time by eliminating, in particular by decanta- .tion, calcium oxalate and other matters.

The invention is further illustrated by the following example which, of course, has no limitative character.

- 100 kgs. of wheat straw dried in air, suitably cut and freed from dust, are placed in 1200 liters of an acid solution containing about 200 kgs. of NOaH; the temperature is gradually raised and in the course of one hour the temperature reaches 70-75" C. After this treatment, the straw has retained and partly transformed about kgs. of NOcI-I; the reaction has given, among other products, a small amount of oxalic acid.

In order to make sure that the free acid that has been retained is fully neutralized, and in i order to render the mass alkaline, it is then necessary to employ about 20 kgs. of lime diluted in 1200liters of water; after one quarter or an hour at 50 C., I obtain about 40 kgs. of soluble calcium nitrate, and about 1 kg. of insoluble calcium oxalate is formed. This lime lye, when evacuated, carries along a great part of these salts, and there remains in the straw that has been treated but about 2,500 kgs. of calcium nitrate, and a small amount of calcium oxalate.

For the third treatment, I employ 1200 liters of a 2.5 sodium bisulfite lye, which reacts upon calcium nitrate and dissolves the nitrated incrustations of straw made soluble by the second treatment. Afterabout one hour of treatment, at boiling-temperature and under atmospheric pressure, the straw is disintegrated; the lye is then evacuated and the paste is defibered washed, purified and bleached through the known means. I thus. obtain a bleached cellulose of very good quality, containing about 83% of alpha cellulose.

The yield of air dried cellulose is about 50 kgs in the case of unbleached cellulose and 45 kgs. in the case of bleached cellulose.

What I claim is:

1. The method of treating a fibrous vegetable material which comprises wetting said material with a nitric acid aqueous solution of a concentration ranging from 8 to 20 B. at atmospheric pressure and at a temperature ranging from 60 to C., draining off the acid solution not retained, either chemically or physically, by said vegetable material, adding a solution of a base of the group constituted by calcium and sodium bases toneutralize the nitric acid physically retained by the vegetable material, draining off the liquor, which contains in solution the nitrate thus formed and collecting it, and treating the remaining cellulosic material with an alkali bisulfite lye under pressure and temperature conditions suitable to remove incrustations.

2. The method of treating a fibrous vegetable material which comprises wetting said material with a nitric acid aqueous solution of a concentration ranging from 8 to 20 B. at atmospheric pressure and at a temperature ranging from 60 to 80 C., draining off the acid solution not retained, .either .chemically or physically, by said vegetable material, adding a calcium base solution to neutralize the nitric acid physically retained by the vegetable material, draining oil the liquor, which contains in solution the calcium nitrate thus formed and collecting it, and treating the remaining cellulosic material with an alkali bisulfite lyeunder pressure and temperature conditions suitable to remove incrustations.

3. .The method of treating a fibrous vegetable material which comprises thoroughly impregnating said material with a nitric acid aqueous solu- 5 tion of a concentration ranging from 8 to 20 B. at atmospheric pressure and at a temperature ranging rrpmso to 80 C., draining oi! the acid solution not retained, either chemically or physically, by said vegetable material, adding a lime solution to neutralize the nitric acid physically retained by the vegetable material, draining 01f the liquor which contains in solution the calcium nitrate thus formed and collecting itf and treating the remaining cellulosic GILBERT BAUDINIERE.

REFERENCES CITED The following references are of record in the file of this patent:

, l material with a sodium bisulfite lye under pres- 6 UNITED STATES PATENTS Number Paper World. June 19;

Name Date Leaver Mar. 3, 1925 Bradley Apr. 20, 1926 Boistesselin July 7, 1931 Acree July 28, 1931 Heimann Jan. 5, 1932 Kipper Jan. 8, 1935 Miles Mar. 23, 1937 OTHER REFERENCES 

1. THE METHOD OF TREATNG A FIBROUS VEGETABLE MATERIAL WHICH COMPRISES WETTING SAID MATERIAL WITH A NITRIC ACID AQUEOUS SOLUTION OF A CONCENTRATION RANGING FROM 8 TO 20* BE. AT ATMOSPHERIC PRESSURE AND AT A TEMPERATURE RANGING FROM 60 TO 80* C., DRAINING OFF THE ACID SOLUTION NOT RETAINED, EITHER CHEMICALLY OR PHYSICALLY, BY SAID VEGETABLE MATERIAL, ADDING A SOLUTION OF A BASE OF THE GROUP CONSTITUTED BY CALCIUM AND SODIUM BASES TO NEUTRALIZE THE NITRIC ACID PHYSICALLY RETAINED BY THE VEGETABLE MATERIAL, DRAINING OFF THE LIQUOR, WHICH CONTAINS IN SOLUTION THE NITRATE THUS FORMED AND COLLECTING IT, AND TREATING THE REMAINING CELLULOSIC MATERIAL WITH AN ALKALI BISULFITE LYE UNDER PRESSURE AND TEMPERATURE CONDITIONS SUITABLE TO REMOVE INCRUSTRATIONS. 